Segmented resonant antenna for radio frequency inductively coupled plasmas

ABSTRACT

An ion shower system is disclosed and comprises a plasma source operable to generate source gas ions within a chamber. The plasma source further comprises a plurality of conductor segments and a plurality of capacitors, wherein the conductor segments are serially connected through the plurality of capacitors. The plasma source further comprises an antenna drive circuit coupled to the plurality of conductor segments that provides power to the conductor segments and capacitors at a predetermined frequency. The ion shower system also comprises a source gas inlet that provides a source gas to the chamber. The conductor segments, capacitors and antenna drive circuit cooperatively provide energy to charged particles in the chamber, thereby energizing the charged particles and generating a plasma comprising source gas ions and electrons within the chamber due to ionizing collisions between the energized charged particles and the source gas.

FIELD OF THE INVENTION

The present invention relates generally to ion shower systems, and moreparticularly to a system and method for performing SIMOX typeimplantation using an ion shower system.

BACKGROUND OF THE INVENTION

Silicon-on-insulator (SOI) technology offers particular advantages inthe fabrication of certain integrated circuit (IC) devices, as well asin other applications. Among these advantages is higher performance overconventional devices, reduced power consumption, improved radiationimmunity, smaller die size, and process simplification. Tools thatfacilitate the economical production of high quality starting material,or wafers, to the SOI community can help drive the technology to greateracceptance.

Several different techniques presently exist to form SOI type wafers.One conventional process employs the implantation of hydrogen into thewafer to assist in fracturing a wafer assembly comprising a wafer with asurface-deposited oxide layer bonded to another silicon wafer. Theimplanted hydrogen preferentially allows the assembly to fracture alonga plane parallel to the oxide surface, resulting in a thin surfacesilicon-on-oxide sandwich on a silicon substrate.

Another conventional technique employed to form SOI wafers is atechnique called “separation by implanted oxygen” (SIMOX). In the SIMOXprocess, a thin layer (e.g., about 1,000-3,000 Angstroms) of amonocrystalline substrate is separated from the bulk of the substrate byimplanting oxygen ions into the wafer to form a buried dielectric layer(BOX). Such implantation conventionally is performed with animplantation dose of about 1×10¹⁸ to about 3×10¹⁸ oxygen ions/cm², andthe resultant buried dielectric layer ranged in thickness from about1,000-5,000 Angstroms. The SIMOX process thus results in aheterostructure in which a buried silicon dioxide layer serves as aneffective insulator for surface layer electronic devices.

Traditional SIMOX processing employs an ion implantation system, whereina pencil-shaped beam or a ribbon-shaped beam is generated, mass analyzedand directed toward an end station. The end station is a batch-type endstation, wherein a plurality of workpieces or wafers reside and spinabout an axis. In pencil-type beams 10, wherein the beam width issubstantially smaller than the size of the wafer 12, a magnetic scannerapparatus is employed to radially scan the beam with respect to theendstation, such that as the wafers spin 14 about the axis, the oxygenion beam scans 16 across each of the wafers, as illustrated in prior artFIG. 1A. The above solution requires a scanning mechanism and associatedcontroller. In addition, as can be seen in prior art FIG. 1B, however,such a scanning process is not trivial; rather since some portions ofthe beam will be traversed twice per full scan, while other portions arescanned only once, a moderately sophisticated scan and rotation controlarchitecture must be controlled to emulate a typically desirableLissajou pattern.

When employing a ribbon-shaped beam 20 as illustrated in FIGS. 2A and2B, 20 the width 22 of the beam is typically larger than the diameter 24of the wafer 12, and thus many of the above challenges associated withthe above conventional scanning process are avoided. Use of aribbon-beam 20, however, has challenges with respect to wafer cooling.Typically, a SIMOX process is controlled modestly stringently at about600 C such that the implantation self-anneals to repair the lattice ofthe wafer. Thus challenges exist to balance the beam power withradiative cooling that is employed as the wafer spins about the axis. Inaddition, although the ribbon-beam does not have to scan across thewafer, since the wafers are off-axis the current density seen byportions of the wafer further away from the axis decreases by 1/r,wherein r is the distance from the axis to the portion of the wafer atissue. Thus, non-uniform implantation and thermal effects may occurunless additional control is employed. Varying the current density ofthe ribbon-beam along its width to accommodate for the above variationis rather challenging and requires further system complexities.

SUMMARY OF THE INVENTION

The following presents a simplified summary in order to provide a basicunderstanding of one or more aspects of the invention. This summary isnot an extensive overview of the invention, and is neither intended toidentify key or critical elements of the invention, nor to delineate thescope thereof. Rather, the primary purpose of the summary is to presentsome concepts of the invention in a simplified form as a prelude to themore detailed description that is presented later.

The present invention is directed to an ion shower system for use inimplantation of ions into a workpiece. The system is particularlyadvantageous for use in SIMOX applications, wherein a high oxygenfraction and high beam current uniformity is desired.

According to one aspect of the present invention, the ion shower systemcomprises a plasma source associated with a chamber. The system furthercomprises a workpiece support structure associated with a top portion ofthe chamber, wherein the workpiece support structure is operable tosecure a workpiece such as a 300 mm silicon wafer. The support structuresecures the workpiece such that an implantation surface thereof isoriented downward toward the extraction assembly. The plasma sourcereceives an input source gas (e.g., oxygen) and generates a plasmausing, for example, RF power. The source and/or chamber are configuredsuch that the generated plasma is azimuthally symmetric within thechamber. The extraction assembly is operable to extract the uniformplasma vertically toward the secured workpiece for implantation thereof.The upside-down design advantageously facilitates reduced contaminationsince any contaminants suspended within the plasma fall away from theworkpiece upon a deactivation of the plasma. The configuration alsoadvantageously facilitates use of evaporative cooling of the workpieceby configuring an evaporative cooling structure on a top portion of theworkpiece support structure, wherein such cooling can be provided in aspatially uniform manner across the workpiece.

In accordance with another aspect of the present invention, the ionshower system comprises a chamber having at least two grounding rodstherein. The chamber walls are preferably coated with silicon to reducecontamination, however, when oxygen is the source gas, the oxygen reactswith the silicon chamber sidewalls, causing the walls to oxidize andbecome an insulator. Thus the grounding rods in the chamber serve tocollect excess electrons during ion extraction. The grounding rods aresilicon coated and while one is biased to a ground potential, the otheris biased negative such that ions in the chamber sputter off any oxidethat has formed on the grounding rod. The ground potential and negativepotential are then switched back and forth between the rods to maintaina path for excess electrons while preventing the rods from undulyoxidizing.

According to still another aspect of the present invention, the ionshower chamber comprises a radial confinement system operable toradially confine the plasma within the chamber. The confinement system,in one example, comprises a magnetic device that generates multi-cuspmagnetic fields that serve to prevent the plasma from reaching thechamber sidewalls. In one example, the magnetic device comprises aplurality of independently drivable coils that permit the strength ofresulting multi-cusp magnetic fields to be individually controlled fortuning. The confinement system aids in the generation and maintenance ofazimuthally symmetric plasma within the chamber, which advantageouslyfacilitates uniform beam current at the workpiece.

According to yet another aspect of the present invention, the ion showersystem comprises an extraction assembly that comprises a multi-electrodearrangement. A first electrode, a plasma electrode, is associated withthe chamber and is biased at the same potential as the plasma. Theplasma electrode has a plurality of extraction apertures within anextraction region, however, due to the high-density plasma within thechamber, the plasma electrode may have a transparency of as low as 10%.A second electrode, an extraction electrode, is biased negatively withrespect to the plasma electrode to form an electrostatic fieldtherebetween and extract positive ions from the chamber. The extractionelectrode has a plurality of extraction apertures associated therewiththat are aligned substantially with respect to the extraction aperturesin the plasma electrode. The extraction electrode further comprises aplurality of interstitial pumping apertures that serve to pump excessneutral source gas therethrough and substantially reduce a pressureassociated with the extraction assembly. The reduced pressure improvessystem reliability by reducing discharges within the chamber due toincreased pressure.

The pattern of extraction electrodes may be spatially uniform oralternatively may vary to provide compensation for any plasmanon-uniformities. For example, if the plasma density within the chamberdecreases slightly azimuthally (about the periphery of the chamber), theextraction aperture density can be increased peripherally to providecompensation, thereby further improving beam current uniformity at theworkpiece.

According to still another aspect of the present invention, the ionshower system comprises an RF antenna system for generating a plasmawithin the system chamber. A neutral source gas, for example, oxygen, isinjected into the chamber. The RF antenna system produces RF electricfields that excite charged particles in the chamber that cause a plasmato be generated due to collisions of the accelerated charged particleswith the neutral source gas atoms. The antenna system comprises aplurality of conductive loop antenna segments serially coupled togetherthrough capacitors. The arrangement reduces an undesirable non-uniformcapacitive field component by a factor of N, wherein N is the number ofconductive loop antenna segments. The reduction in the capacitive fieldcomponents advantageously facilitates an improvement in plasmauniformity within the chamber. The plurality of conductive loop segmentspreferably are configured in an azimuthally symmetric arrangement suchthat, to the extent that any non-uniformity exists for each segment,such non-uniformity is itself azimuthally symmetric within the chamber.

To the accomplishment of the foregoing and related ends, the followingdescription and annexed drawings set forth in detail certainillustrative aspects and implementations of the invention. These areindicative of but a few of the various ways in which the principles ofthe invention may be employed. Other aspects, advantages and novelfeatures of the invention will become apparent from the followingdetailed description of the invention when considered in conjunctionwith the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a plan view diagram illustrating a convention endstation anda scanning mechanism for implanting ions into a plurality of workpiecesusing a pencil-type beam;

FIG. 1B is a plan view diagram illustrating in greater detail theconventional scanning of a wafer of FIG. 1A, wherein a scanning pathresults in portions receiving differing doping concentrations;

FIGS. 2A-2B are plan view diagrams illustrating a conventionalendstation and a scanning mechanism using a ribbon-type ion beam;

FIG. 3 is a side elevation view of an ion shower system in accordancewith one or more aspects of the present invention;

FIG. 4 is a graph illustrating an oxygen fraction obtained using the ionshower system of FIG. 3 in accordance with one aspect of the presentinvention;

FIG. 5 is a side elevation view of a portion of the ion shower system ofFIG. 3, illustrating in greater detail a chamber portion of varioussubsystems associated therewith;

FIG. 6 is a sectional view illustrating a portion of the ion chamberhaving plasma flow lines associated therewith;

FIG. 7 is a side elevation view of an extraction assembly in accordancewith one aspect of the present invention;

FIGS. 8-9 are plan views of a plasma electrode and an extractionelectrode, wherein the plasma electrode has extraction apertures and theextraction electrode has extraction apertures and interstitial pumpingapertures associated therewith;

FIGS. 10-11 are fragmentary sectional views of extraction assemblies,wherein the assembly of FIG. 10 has extraction apertures and theassembly of FIG. 11 has both extraction apertures and interstitialpumping apertures associated therewith;

FIG. 12 is a plan view of an antenna assembly employed for generating aplasma in the ion shower system of FIG. 3 in accordance with anotheraspect of the present invention;

FIG. 13 is a side elevation view taken along line 13-13 of FIG. 12illustrating the antenna assembly of the present invention; and

FIG. 14 is an exemplary schematic diagram illustrating an antennaassembly similar to that of FIGS. 12 and 13, according to an aspect ofthe present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will now be described with reference to thedrawings wherein like reference numerals are used to refer to likeelements throughout. The illustrations and following descriptions areexemplary in nature, and not limiting. Thus, it will be appreciated thatvariants of the illustrated systems and methods and other suchimplementations apart from those illustrated herein are deemed asfalling within the scope of the present invention and the appendedclaims. The present invention pertains to an ion shower system for usein implantation of ions into a workpiece. The present invention may findparticular application in the implantation of oxygen ions, for example,in a SIMOX type process to form SOI wafers for use in semiconductorprocessing. Other applications, however, may be available and arecontemplated as falling within the scope of the present invention.

Turning now to the figures, FIG. 3 illustrates an ion shower system 100in accordance with one aspect of the present invention. The ion showersystem 100 comprises a chamber 102 supported by a plurality of supportstructures 104. The chamber 102 may reside within a cabinet enclosure106, a portion 108 of which may house a feed gas (not shown) such asoxygen, one or more power supplies (not shown), as well as othercomponents as needed.

The chamber 102 has a bottom portion 110, a top portion 112 and sideportions 114, respectively. In one example, the side portions 114comprise a cylinder, thereby making the chamber radially or azimuthallysymmetric. An extraction assembly 116 is associated with the top portion112 of the chamber 102, and couples a process chamber 118 containing aworkpiece (not shown) therein. The workpiece is secure within theprocess chamber 118 with a clamping system 120, such as an electrostaticclamp, however, other mechanisms may be employed and are contemplated asfalling within the scope of the present invention. As will be discussedin greater detail infra, the process chamber 118 may further include apump 122 for removal of neutral gas from the process chamber to maintaina desired pressure environment.

The chamber 102 further comprises an inlet gas port 124, and in thepresent example, the port is associated with the bottom 110 thereof.Also associated with the bottom portion 110 of the chamber 102 is anantenna system 126 comprising multiple single loop antennas coupledtogether. As will be discussed in greater detail below, the antennasystem 126 is coupled to an RF source (not shown) and provides RFexcitation for plasma generation within the chamber, wherein theexcitation is azimuthally symmetric. Such excitation facilitates plasmauniformity within the chamber 102 that advantageously facilitates beamcurrent uniformity at the workpiece.

The chamber 102 further comprises a grounding system 128, for example, aplurality of grounding rods that operate to collect electrons while ionsare extracted from the chamber in order to maintain plasma stability.Further, the chamber 102 further comprises a plasma confinement system130 associated with at least the side portions 114 of the chamber. Inone example, the plasma confinement system 130 comprises a plurality ofmulti-cusp magnets that generate multi-cusp magnetic fields that extendinto the chamber. The multi-cusp magnetic fields operate to confine theplasma radially and aid in further facilitating azimuthal plasmasymmetry within the chamber 102.

According to one aspect of the invention, the ion shower system 100 hasan upside-down design, wherein the extraction assembly 116 and workpieceare associated with the top portion 112 of the chamber 102. In the abovemanner, the generated ions within the chamber 102 are extractedvertically upwards via the extraction assembly 116 and with theworkpiece oriented facing down, the ions are implanted therein. Such anarrangement advantageously provides a reduction in contamination at theworkpiece. For example, any contaminants suspended in the plasma willfall to the bottom portion 110 of the chamber 110 upon deactivation ofthe plasma due to the influence of gravity. Thus upon deactivation, anycontaminants fall away from the workpiece as opposed to falling towardsthe workpiece in a conventional system wherein the workpiece is situatedbelow the chamber. Further, the vertical orientation of the system 100advantageously aids in beam uniformity at the wafer. For example, onefactor in achieving beam uniformity is the alignment and focus ofbeamlets through the extraction assembly 116. The vertical orientationhighlighted above allows the extraction electrodes to also be verticallyoriented, and such orientation avoids or reduces cantilevering forcesthat may negatively impact alignment when assembling or maintaining thesystem.

Another advantageous feature associated with the upside-down design isthe configuration facilitates use of an evaporative cooling unit (notshown) in association with the clamping system 120. Due to the high beamcurrent, cooling the workpiece is an important feature. Evaporativecooling is more effective than mere convective cooling, and with theupside-down design, an evaporative cooling system may reside on top ofthe clamping system 120 (e.g., an electrostatic clamp). For example,water may be employed as the cooling medium and overlay the entireworkpiece portion of the clamping system 120. As the water heats up andboils, steam escapes and energy is dissipated uniformly about theworkpiece. Such a mechanism would not operate in the same uniform mannerif the process chamber 118 was oriented to the chamber 102 in anothermanner.

The chamber 102 is a large volume chamber, for example, having adiameter of about 80 cm and a height of about 60 cm. Such a volume isdesirable for oxygen implantation of a workpiece such as a 300 mm (30cm) semiconductor wafer. By having a diameter substantially larger thanthe workpiece diameter, plasma uniformity within the chamber will extendbeyond the extent of the workpiece, thereby facilitating a high beamuniformity thereat. In addition, a large chamber volume advantageouslyaids in achieving a large O₊/O₂₊ fraction, for example, greater than 98%when using the ion shower system for a SIMOX type process.

It was found that various wall reactions occur within the chamber 102,for example:

-   -   O₊→O,    -   O₂₊→O₂,    -   2O→O₂.        In particular, the wall recombination (2O→O₂) is particularly        undesirable since such recombination negatively impacts        (decreases) the O₊/O₂₊ fraction. By having the large volume        chamber 102, the volume to surface ratio is large and thus        although surface recombination still occurs, such recombination        is subsumed by the O₊ ions generated within the volume.        Operating the above configuration at about 7.5 kW power and        about 0.5 mTorr pressure, a plasma density of about 1×10¹¹/cm²        is achieved with an O₊/O₂₊ ratio substantially greater than 98%,        as illustrated in FIG. 4.

The ion shower system 100 of FIG. 3 is not mass analyzed, thus thepresent invention contemplates coating the interior of the chamber 102with silicon. Thus any “contamination” is silicon, and since the SIMOXprocess is performed into silicon, such contamination is permitted andis not a problem. The source interior, however, being silicon, willoxidize and become SiO₂ after exposure to oxygen plasma. SiO₂ is not aproblem with respect to contamination in a SIMOX process, however, asthe interior surface becomes an electrical insulator, the surface cannotbe employed as a ground reference to the plasma. Consequently, as ionsare extracted, no path exists for excess electrons, and thus the plasmawill tend to float negative and at some point eventually prevent ionextraction toward the workpiece.

Therefore the ion shower system 100 of the present invention employs thegrounding system 128 employing a plurality of grounding rods within thechamber 102. In one example, two silicon coated grounding rods 128 areemployed, wherein the silicon coating is doped to make the rods moreconductive. Preferably, the rods are doped with a P-type dopant such asBoron since semiconductor wafer starting material is typically a lightlydoped P-type substrate.

In a preferred aspect of the present invention, one of the rods 128 isgrounded and serves to collect excess electrons during ion extraction,while the second rod is biased negatively. Since the grounding rods 128will tend to oxidize due to the presence of oxygen within the chamber102, the negative bias will cause the biased rod to be sputtered cleanby the plasma, thereby removing any oxide build-up thereon.Subsequently, the role of the rods is reversed, and the previouslybiased rod is grounded while the previously grounded rod becomesnegatively biased. In the above manner the ground reference ismaintained and such sputtering of the rods does not result inunacceptable contamination at the workpiece.

In accordance with one exemplary aspect of the present invention, asquare wave voltage is applied to the two grounding rods 128, whereinthe voltage to each is approximately 180 degrees out of phase with oneanother. In the above manner, at any instant, at least one of thegrounding rods is grounded and acts to collect electrons during ionextraction while the other, negatively biased rod is being sputtered.Alternatively, a timing arrangement may vary, wherein at least one ofthe rods is grounded during the extraction process and any suchvariation is contemplated as falling within the scope of the presentinvention.

Turning now to FIG. 5, the chamber 102 is illustrated in greater detail.As discussed above in conjunction with FIG. 3, the chamber 102 includesa plasma confinement system 130 along at least the side portion(s) 114thereof. The plasma confinement system 130 is configured and adapted toprovide containment of plasma along the periphery of the chamber. In oneexample where the chamber 102 is a cylinder, the system 130 operates toprovide radial confinement along a vertical height 140 thereof. Suchradial confinement may operate to advantageously facilitate azimuthalplasma uniformity within the chamber 102. In one example, theconfinement system 130 operates to provide multi-cusp magnetic fieldsalong the periphery or side portions 114 of the chamber 102. As may beappreciated, the multi-cusp magnetic field lines 144 operate to providea substantial impedance to the plasma since the plasma tends to notcross the magnetic field lines. Thus the multi-cusp magnetic field linesextending into the chamber provide radial plasma containment andcontribute to azimuthal plasma uniformity therein.

Any type of magnetic device may be employed to generate such fields, forexample, an alternating pattern of permanent magnets having north andsouth poles that each encircle the chamber. Alternatively, and morepreferably, the plasma confinement system 130 comprises a plurality ofcoils 142, wherein the coils are an alternating pattern encircling thechamber 102 such that neighboring coils have currents traveling inopposite directions (e.g., being oppositely wound), resulting inmulti-cusp magnetic fields 144. In such an example, the coils each maybe independently driven (drive circuit not shown), therebyadvantageously providing tunability in the multi-cusp fields to maximizeplasma uniformity within the chamber 102.

In addition, the plasma confinement system 130 may extend to the bottomportion 110 of the chamber 102, as illustrated in FIG. 5. In such amanner, the multi-cusp magnetic fields may extend up into the chamberinterior from the bottom portion for improved plasma confinement anduniformity, as exemplified by the field lines 146. In one example, theplasma confinement system 130 on the bottom portion 110 of the chamber102 comprises a plurality of permanent magnets have north and southpoles associated therewith, wherein the magnets are arranged tofacilitate plasma uniformity azimuthally, and any such arrangement iscontemplated as falling within the scope of the present invention.

FIG. 6 illustrates exemplary plasma confinement within the chamber 102employing the exemplary arrangement of FIG. 5. FIG. 6 illustratescontour lines representing regions of constant magnetic field magnitudewithin the chamber in cross section. Note that in FIG. 6, the sideportion 114 of the chamber 102 has the multi-cusp magnets 142 thatprovide multi-cusp fields in a region 150 extending radially into thechamber. The magnetic field strength is illustrated by contour lines 145of constant field magnitude, wherein close to the wall, 145 a, the fieldis substantially high (e.g., about 1000 Gauss) whereas the field furtherin toward the chamber interior, 145 b, is substantially lower (e.g.,about 20 Gauss). Thus in the center of the chamber the plasma is free tomove in a generally magnetic field free zone (e.g., magnetic fieldintensity is generally negligible). The multi-cusp fields aid in plasmacontainment, and in conjunction with the magnetic field free zones,facilitate plasma uniformity within the chamber. Such plasma uniformityadvantageously results in beam uniformity at the workpiece (e.g., a beamcurrent variation across a 300 mm wafer of less than 2%).

According to another aspect of the present invention, the extractionassembly 116 operates to extract oxygen ions from the chamber 102 anddirect such ions toward a workpiece for implantation thereof at a givenenergy level. One exemplary extraction assembly of the present inventionis illustrated in greater detail in FIG. 7, and comprises a pentode typeelectrode arrangement, wherein five electrodes 202-210 are aligned andspaced from one another via dielectric spacers 212. A first electrode202 is the plasma electrode and contacts and attaches to the top surface112 of the chamber 102. The plasma electrode 202 is biased with respectto the other electrodes 204-210, but floats with respect to the plasma(e.g., at 120 kV with respect to the workpiece that is typicallygrounded) within the chamber 102. Each of the electrodes 202-210 have anextraction region 214 through which a plurality of ion beamlets pass,and the extraction region has a diameter or size 216 that is at least aslarge as a diameter of the workpiece and is preferably larger in orderto ensure beam uniformity thereat.

The second electrode 204 comprises the extraction electrode and isbiased at a voltage less than that of the plasma electrode 202 (e.g.,105 kV). The negative relative potential with respect to the plasmacreates an electrostatic field operable to extract positive ions (e.g.,oxygen ions) out of the plasma chamber 102. The third, fourth and fifthelectrodes 206, 208, and 210 comprise an auxiliary electrode, asuppression electrode, and a ground electrode, respectively. The groundelectrode 210 is biased at the same voltage as the workpiece (e.g., 0V),while the suppression electrode 208 is biased at a voltage that isnegative with respect to the ground electrode (e.g., −20 kV). Thesuppression electrode 208 operates to prevent electrons from a plasmalocal the workpiece (operating as an electron shower for chargeneutralization at the workpiece) from entering the extraction assembly116. The auxiliary electrode 206 operates to step down the voltage andis biased at an interim voltage (e.g., 40 kV) between the extractionelectrode 204 and the suppression electrode 208, respectively.

The plasma electrode 202 has a plurality of extraction aperturesassociated therewith, as illustrated in FIG. 8 and designated atreference numeral 220. FIG. 8 is not drawn to scale, but rather isprovided for purposes of illustration. The extraction apertures 220 arerelatively small (e.g., about 3-4 mm in diameter), whereas the entireregion diameter 214 is greater than 300 mm. The extraction aperturecenters are spaced apart from one another (e.g., about 2 cmcenter-to-center), in a uniform manner, in one example, with spacingsuch that the electrode transparency is about 10%. Use of a high densityplasma in the plasma chamber 102 permits the low transparency and alsoallows the plasma electrode 202 to be cooled via fluid ports/channelstherethrough (not shown), if desired.

Alternatively, the extraction apertures may be spaced from one anotherin a non-uniform manner, with such non-uniformity configured to providecompensation for any non-uniformity in the plasma within the chamber102. For example, if the plasma within the chamber 102 tends to “falloff” azimuthally, wherein a plasma density along an outer peripheryregion (that may correspond to the workpiece periphery) is about 5-10%less than a plasma density at the center, the density of extractionapertures 220 may be made greater along an outer periphery of theelectrode 202 (and other electrodes). In the above manner, more ions areextracted along the outer periphery, leading to increased ion beamuniformity spatially at the workpiece. Any variation in extractionaperture configuration to compensate for plasma non-uniformities may beemployed and is contemplated as falling within the scope of the presentinvention.

The extraction electrode 204 and other electrodes 206, 208 and 210 alsohave extraction apertures associated therewith that are aligned withrespect to the extraction apertures 220 of the plasma electrode. Inaddition, the electrodes 204-210 have interstitial pumping aperturesassociated therewith, as illustrated in FIG. 9 and designated atreference numeral 222. As will be further appreciated below, theinterstitial apertures 222 substantially improve extraction reliabilityby substantially reducing the pressure within the extraction assembly116, thereby preventing discharges from undesirably shorting out theelectrodes.

The inventors of the present invention identified that the high pressurewithin the chamber 102 could provide some operational disadvantages. Forexample, for a relatively high pressure in the chamber 102 (e.g., about1 mTorr), a substantial amount of charge exchange occurs due to the ionbeamlets colliding with neutral gas. Adding a pump in the workpiecechamber 118 operates to pump out the neutral gas passing through theextraction assembly 116 but not help substantially to reduce thepressure because transparency through an initial extraction assembly islow and gas conduction to the electrode sides is found to be relativelylow, as illustrated in FIG. 10. In such an instance, the pressuredifferential between the chamber 102 and a region 230 between theelectrodes 202′ and 204′ of FIG. 10 is relatively low (e.g., about 0.1mTorr) which can disadvantageously result in discharges that can shortout the extraction assembly.

The electrodes 204-210 of the present invention employ the interstitialpumping apertures 222 in addition to the extraction apertures 220, asillustrated in FIGS. 9 and 11, that substantially lower the pressure inthe extraction assembly (e.g., 1-2 orders of magnitude compared to thechamber pressure). Consequently, gas “constriction” exists at the plasmaelectrode 202, but substantial conductance is provided in the assembly116 thereafter due to the pump 122 in the workpiece chamber 118 removingthe neutral gas passing through the interstitial pumping apertures 222.

As illustrated in FIG. 11, the interstitial pumping apertures 222 arenot aligned with the extraction apertures 220. Thus, the oxygen beamlets234 still are extracted through the extraction apertures 220 withoutbeing impacted, while the interstitial pumping apertures 222 readilyallow a plurality of neutral gas conducting paths through the assembly116. Pumping of such neutral gas through the extraction assembly 116 isimportant to keep the pressure within the plasma chamber 102 fromgetting too high and creating discharges. Although the interstitialpumping apertures 222 in FIG. 11 are illustrated as being aligned withrespect to one another, alternatively, such apertures may be staggeredwith respect to one another and such a variation is contemplated asfalling within the scope of the present invention.

In addition, it should be noted that the extraction assembly can beconfigured in a variety of ways to regulate an amount of ion beamletoverlap at the workpiece to adjust an amount of beam uniformity thereat.For example, as illustrated in FIG. 11, a potential difference betweenthe plasma electrode 202 and the extraction electrode 204 creates anelectrostatic field that pushes electrons in the plasma chamber 102 awayfrom the extraction apertures 220 in the plasma electrode 202, resultingin a plasma sheath 232 that appears like an inverted meniscus. Themagnitude of the voltage difference dictates the magnitude of theelectrostatic field, thereby impacting a shape of the plasma sheath. Thesheath 232 acts as a lens for the ion beamlets 234, thereby affecting afocus of each of the beamlets. As illustrated in FIG. 11, the plasmasheath 232 impacts a focusing of the beamlets, thereby allowing theextraction apertures 220 of each of the electrodes (although stillaligned) to be uniquely sized so as to pass the ion beamlets whileconcurrently blocking contaminants.

The beamlets 234 exit the last electrode (the ground electrode 210), anddue to the plasma sheath optics, each of the beamlets is slightlydiverging. The workpiece (not shown) is spaced away from the groundelectrode 210 a predetermined distance such that the beamlets 234overlap at the workpiece surface for beam uniformity thereat. In SIMOXtype processing, it is desirable to obtain an oxygen ion beam uniformityof <1% variation at the workpiece. The inventors of the presentinvention have determined that such uniformity may be obtained if atleast three (3) beamlets 234 overlap at the workpiece surface. That is,a predetermined distance is provided (in accordance with the plasmasheath 234) such that an edge of one beamlet touches a beamlet center atleast two beamlets away.

Turning now to another aspect of the present invention, the antennasystem 126 will be described in conjunction with FIGS. 12-14. FIG. 12illustrates a plan view of the antenna system 126 taken from a positioninside the plasma source chamber 102 of FIG. 3, while FIG. 13 is a sideelevation view taken along line 13-13 of FIG. 12. And lastly, FIG. 14 isa schematic diagram illustrating an effective electrical circuit of theantenna system 126 of FIGS. 12-13.

Referring to FIGS. 12 and 13, the antenna system 126 comprises a base302, for example, associated with a bottom portion 110 of the chamber102 upon which a plurality of antenna conductors 304 reside. The antennaconductors 304 are serially coupled together through capacitors 306associated with the base 302, and two of the conductors are coupledexternally to an antenna drive circuit 308 (FIG. 14), through an RFcoupling mechanism 310. In addition, the base 302 has an aperture 312within a central portion thereof, wherein an inlet source gas feed 314extends therethrough to form a portion of the inlet gas feed port 124 ofFIG. 3. The inlet source gas fed 314 provides a neutral gas (e.g.,oxygen) in the chamber 102 for ionization thereof to form a plasma, aswill be discussed further below.

In accordance with one aspect of the present invention, the antennadrive circuit may comprise an integrated power oscillator RF source suchas that described in U.S. Pat. No. 6,305,316 and assigned to theassignee of the present invention, which is hereby incorporated byreference in its entirety. In such an example, the integrated poweroscillator employs characteristics of the RF segmented antenna withinthe oscillator tank circuit. Such incorporation advantageouslyfacilitates high-speed ignition of the plasma that is important, in somecases, because when the system is activated the ion beam is immediatelystriking the workpiece and no time exists for a manual tuning of thetank circuit. The integrated power oscillator advantageouslyautomatically adapts to the change in the plasma impedance during plasmaignition.

The antenna conductors 304 are arranged in an azimuthally symmetricfashion about the base 302 as illustrated in order to generate anazimuthally symmetric plasma within the chamber 102. Such uniformity ofthe plasma operates to provide advantageously a uniform beam current atthe workpiece. Although six conductors 304 are illustrated in FIG. 12,it should be understood that any number “N” of such conductors may beemployed and such variations are contemplated by the present invention,wherein “N” is an integer greater than 1. The antenna system 126 of thepresent invention generates a plasma via inductive coupling, and thearrangement provides a substantial reduction in variation along theantenna elements compared to conventional arrangements without a needfor a faraday shield.

The RF antenna system 126 of the present invention operates in thefollowing manner. A time dependent current is generated in theconductors 304 via the antenna drive circuit 308. The time-varyingcurrent produces a magnetic field that surrounds the conductive elementsin accordance with Faraday's law. Because the current is time-varying,the produced magnetic field is a time-varying field. In accordance withMaxwell's equations, the time-varying magnetic field induces atime-varying electric field normal thereto, wherein the time-varyingelectric field extends along a direction of the conductors 304 anddecays as the field extends away therefrom. This time-varying electricfield is referred to as the inductive electric field component since itis induced from the time-varying magnetic field.

The time-varying inductive electric field accelerates charged particlessuch as electrons near the antenna conductors 304. Further, the antennaelements are configured such that the velocity of the acceleratedcharged particles is sufficient so that the charged particles movethrough the region associated with a conductor in a time that is shortcompared to the period (T) of the time-varying current. Consequently,the charged particles see a substantially steady field as it travelsalong the conductor 304. Therefore the time-varying electric field“heats” the charged particles that then have sufficient energy to ionizethe source gas atoms within the chamber 102 upon collision therewith.The ionizing collisions operate to generate the plasma and such plasmageneration is substantially azimuthally symmetric in accordance with theconfiguration of conductors 304 about the base 302.

In addition, the current generated and flowing through the conductors isproduced by an RF voltage (e.g., 800V peak to peak). Therefore theconductor elements 304 have a voltage across each element that spatiallyvaries along a length of the element. Consequently, the varying chargedistribution along an element 304 produces an electrostatic field thatextends from the conductors outwardly and the strength of the fieldvaries spatially along the length of the element. This electrostaticfield component is referred to as the capacitive field component.Because this field is not uniform, the contribution of this fieldcomponent to plasma generation is non-uniform, and thus it is desirablefor this electric field component to be reduced as much as possible.

Some conventional antenna designs employ a faraday shield to block orminimize the capacitive field component. In such a solution, the faradayshield is placed between the conductors and the plasma, however, such asolution increases circuit losses and is practically difficult toconfigure since the conductors in the present system are immersed in theplasma. The antenna system 126 of the present invention overcomes thedisadvantages of the prior art and provides a structure thatsubstantially reduces the capacitive field contribution of the systemwithout use of a faraday shield, as will be further appreciated below.

The antenna design 126 of the present invention divides whatconventionally was a single conductor into “N” conductor segments 304(e.g., N=6 as illustrated in FIG. 12, or N=8 as illustrated in FIG. 14),wherein each conductor segment 304 is DC-isolated from one another, butin RF-series via capacitors 306. Such an arrangement reduces the peakcapacitive electric field component by a factor of N. Referring to FIG.14, preferably the values of L (associated with the conductors 304), C(the capacitors), and f (the frequency of the drive signal from theantenna drive circuit 308) are selected so that the magnitude of thereactance of the inductive component (2πfL) is equal to the magnitude ofthe reactance of the capacitive component (½πfC). In the above manner, aresonant circuit exists, in which the voltage drop across each inductiveelement L is equal and opposite to the voltage drop across eachcapacitive element C. Thus, the maximum voltage drop is reduced by afactor of “N” compared to the case of N inductive elements in serieswithout capacitive elements. Also, although some variation still occursalong a length of an individual component 304, it is N times smallerthan a conventional arrangement, and as illustrated in FIG. 12, suchvariation is itself azimuthally symmetric due to the arrangement of theconductors azimuthally.

The antenna system 126 thus operates to generate a plasma within thechamber 102, wherein the generated plasma is azimuthally symmetric. Theazimuthal symmetry of the plasma advantageously helps to provide aspatially uniform beam current at the workpiece.

Although the invention has been illustrated and described above withrespect to a certain aspects and implementations, it will be appreciatedthat equivalent alterations and modifications will occur to othersskilled in the art upon the reading and understanding of thisspecification and the annexed drawings. In particular regard to thevarious functions performed by the above described components(assemblies, devices, circuits, systems, etc.), the terms (including areference to a “means”) used to describe such components are intended tocorrespond, unless otherwise indicated, to any component which performsthe specified function of the described component (i.e., that isfunctionally equivalent), even though not structurally equivalent to thedisclosed structure, which performs the function in the hereinillustrated exemplary implementations of the invention. In this regard,it will also be recognized that the invention may include acomputer-readable medium having computer-executable instructions forperforming the steps of the various methods of the invention. Inaddition, while a particular feature of the invention may have beendisclosed with respect to only one of several implementations, suchfeature may be combined with one or more other features of the otherimplementations as may be desired and advantageous for any given orparticular application. In accordance with the present invention, theterm “ribbon-like” should be understood to include both a ribbon beamand a scanned pencil type beam. Furthermore, to the extent that theterms “includes”, “including”, “has”, “having”, “with” and variantsthereof are used in either the detailed description or the claims, theseterms are intended to be inclusive in a manner similar to the term“comprising”. Also, the term “exemplary” as utilized herein simply meansexample, rather than finest performer.

1. An ion shower system, comprising: a plasma source operable togenerate source gas ions within a chamber, wherein the plasma sourcefurther comprises: a plurality of conductor segments; a plurality ofcapacitors, wherein the conductor segments are serially connectedthrough the plurality of capacitors, wherein the series arrangement ofconductor segments and capacitors reside within the chamber; an antennadrive circuit coupled to the plurality of conductor segments, andoperable to provide power to the conductor segments and capacitors at apredetermined frequency; and a source gas inlet, wherein the source gasinlet is operable to provide a source gas to the chamber, and whereinthe conductor segments, capacitors and antenna drive circuitcooperatively provide energy to charged particles in the chamber,thereby energizing the charged particles and generating a plasmacomprising source gas ions and electrons within the chamber due toionizing collisions between the energized charged particles and thesource gas; an extraction assembly associated with the chamber, andoperable to extract source gas ions therefrom.
 2. The ion shower systemof claim 1, further comprising a workpiece support structure associatedwith the chamber, and operable to secure the workpiece for implantationthereof of source gas ions from the extraction assembly.
 3. The ionshower system of claim 1, wherein first and last conductor segments ofthe plurality of conductor segments form an input, and Wherein theantenna drive circuit is coupled to the input.
 4. The ion showers systemof claim 1, wherein the conductor segments have an inductive reactanceassociated therewith, and wherein the capacitors have a capacitivereactance associated therewith, and wherein one of the conductors andone of the capacitors form an antenna segment, wherein the inductivereactance and capacitive reactance of the antenna segment are equal atthe predetermined frequency.
 5. The ion shower stem of claim 1, whereinthe plurality of conductor segments and plurality of capacitors form aresonant circuit at the predetermined frequency.
 6. The ion showersystem of claim 1, wherein the antenna, drive circuit comprises anoscillator circuit.
 7. The ion shower system of claim 1, wherein theplurality of conductor segments and capacitors are arranged within thechamber in an azimuthally symmetric fashion, wherein a non-uniformcapacitive electrostatic field component along each conductor segment isrepeated in an azimuthally symmetric fashion.
 8. The ion shower systemof claim 1, wherein the extraction assembly is associated with a topportion of the chamber; and is operable to extract ions vertically fromthe top portion thereof.
 9. The ion shower system of claim 8, furthercomprising a workpiece support structure associated with the top portionof the chamber, and operable to secure the workpiece having animplantation surface orientated facing downward toward the extractionassembly for implantation thereof.
 10. The ion shower system of claim 1,wherein the chamber further comprises a bottom portion and sideportions, and wherein the side portions comprise a plurality ofmulti-cusp magnet devices operable to produce multi-cusp magnetic fieldsthereat to facilitate an azimuthal uniformity of plasma within thechamber.
 11. The ion shower system of claim 10, wherein the multi-cuspmagnet devices comprise electromagnets operable to provide a variationin multi-cusp magnetic field strength at differing positions along theside portions.
 12. The ion shower system of claim 11, wherein theelectromagnets are independently controllable, thereby facilitating atuning of the multi-cusp magnetic fields.